1. Field of the Invention
The present invention relates to a rotation detecting system for use in detecting the rotation and/or the rotational speed exhibited by various machines and also to a bearing assembly equipped with such rotation detecting system.
2. Description of the Related Art
The rotation detecting system of this kind is well known, in which a ring shaped encoder having a plurality of circumferentially arranged magnetic pole pairs (N and S), which serve as, for example, to-be-detected poles, is coaxially mounted on a rotating body so that passage of the magnetic poles of the magnetic encoder can be detected by a magnetic sensor. In the rotation detecting system of this kind, the magnetic sensor generates pulses corresponding to the number of the magnetic pole pairs during one complete rotation of the magnetic encoder.
Also, disclosure has been made on the rotation detecting system, in which the pulses outputted from the magnetic sensor in response to the magnetic pole pairs of the magnetic encoder are multiplied by a multiplying circuit so that the output of pulses, the number of which is greater than the actual number of the magnetic pole pairs, can be obtained for the purpose of increasing the resolution of the detection. (See, for example, the Patent Documents 1 and 2 listed below.)    [Patent Document 1] Japanese Laid-open Patent Publication No. 2001-518608    [Patent Document 2] Japanese Laid-open Patent Publication No. 2002-541485
FIG. 29 illustrates an example of the system for generating the pulses that are multiplied in the manner described above. In this case, two magnetic sensors 40A and 40B arranged at respective positions displaced 90° in phase from each other with one magnetic pole pair taken as one cycle are disposed in face-to-face relation with the magnetic poles of the magnetic encoder, so that output pulses A and B, which are displaced 90° in phase relative to each other can be obtained from those magnetic sensors 40A and 40B. When those output pulses A and B are combined, the multiplied pulses C having a fourfold resolution can be obtained.
However, in such case, if the difference in phase between those output pulses A and B shifts from 90°, an error will occur in pulse width among the multiplied pulses C.
Another system for generating the multiplied pulses is also suggested, in which based on two analog output signals A and B generated from the magnetic sensors 40A and 40B, multiplication may be accomplished by determining the phase o as shown in FIG. 30.
However, even in this case, if the phase o shifts from 90° or the amplitude of each of the analog output signals A and B changes, an error will occur.
In the case of the construction disclosed in the previously mentioned patent publications, a pitch error tends to occur in the multiplied pulses generated, depending on the distribution of magnetic fields of the magnetic encoder and/or circuit characteristics of the multiplying circuit. FIG. 31 illustrates one example of the pitch error in a graph. In such case, there is shown an example in which within the interval between the magnetic poles of one pair, N pieces of multiplied pulses are generated, in which numerals 1, 2, 3 . . . and N in the axis of abscissas represent the number of the multiplied pulses.
As shown therein, if the pitch error occurs between the multiplied pulses so generated, there is a problem in that variation of the rotational speed detected by the use of the multiplied pulses tends to become large even though the resolution of rotation detection increases.